When Should You Choose CROSS APPLY Over INNER JOIN for Optimal Performance?

CROSS APPLY vs. INNER JOIN: When Does CROSS APPLY Shine?

While INNER JOIN is the go-to for most one-to-many relationships, CROSS APPLY offers significant performance benefits in specific situations.

CROSS APPLY's Key Advantage:

Unlike INNER JOIN, CROSS APPLY doesn't mandate a user-defined function (UDF) for the right-hand table. This seemingly small difference becomes critical when handling large datasets requiring partitioning or pagination.

Superiority in Partitioning and Pagination:

Although simple queries using INNER JOIN and CROSS APPLY might yield similar execution plans, CROSS APPLY demonstrates superior efficiency with large datasets needing partitioning or pagination. By eliminating explicit JOIN conditions, it allows the database optimizer to utilize cached intermediate results, leading to faster query execution.

Illustrative Example:

The following query exemplifies CROSS APPLY's advantage over INNER JOIN when iterating multiple times over the right-hand table for each left-hand table row:

/* CROSS APPLY */
SELECT  t1.*, t2o.*
FROM    t1
CROSS APPLY
        (
        SELECT  TOP 3 *
        FROM    t2
        WHERE   t2.t1_id = t1.id
        ORDER BY
                t2.rank DESC
        ) t2o

This query efficiently retrieves the top three t2 records for each t1 row. Replicating this functionality with INNER JOIN would be significantly more complex and less efficient.

Performance Benchmark:

Testing on a 20,000,000-record table showed a dramatic difference: CROSS APPLY executed almost instantaneously, whereas an equivalent INNER JOIN query using a CTE and window function took approximately 30 seconds.

In Summary:

CROSS APPLY is the superior choice when explicit JOIN conditions are unavailable or when dealing with partitioning or pagination of large datasets. Its efficiency in these scenarios makes it an invaluable tool for query optimization.

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